A hidden network of ageing oil tankers — often called the “shadow fleet” or “ghost fleet” — is quietly reshaping global crude oil trade while raising serious concerns about maritime safety, environmental damage and geopolitical stability.

Although President Trump said seizing tankers would be a financial boon, the cost of maintaining just one aging ship has already reached $47 million. By Luke Broadwater and Nicholas Nehamas Reporting from Washington In an effort to crack down on nations it ...